I've expanded on the link I posted last week on Darwin's Atolls.
A beautiful theory has been undone by ugly facts...well, let's just say facts.
How do coral atolls, those shimmering ring shaped islands set against the blue tropical ocean, form? Charles Darwin had pondered this question on the H.M.S. Beagle as she sailed across the Pacific and Indian Oceans in the early 1830s. He famously reasoned that coral colonies begin growing in the shallow waters surrounding oceanic volcanoes. Eventually the volcanoes sink into the ocean, while the coral keep growing upwards. The central area where the volcano existed becomes a deep lagoon, surrounding by a ring of coral reefs.
But he just assumed that the present day corals atolls are growing on a volcanic foundation.
Actually, most are not. Tropical region atolls rest on an earlier generation of coral and limestone. These in turn have grown on an even earlier layer of coral growth and so on through the past few million years.
Darwin at that time didn't know that the climate over the past 2.6 million years had shifted periodically between glacial and inter-glacial phases resulting in sea level changes. The story begins in the Pliocene around 5 million years ago. A long period of sea level stability resulted in reefs and other calcium carbonate sediments accumulating on the shallow waters above sea floor ridges. Thick deposits formed flat topped carbonate banks. The earth's climate began to change around 2.6 million years ago as polar and high latitude ice sheets expanded and then withdrew. As a result, sea level started falling and rising cyclically.
These repeated sea-level fluctuations amplified around half a million years ago. When sea level fell, rain water dissolved the exposed carbonate flats to form an uneven karst topography comprising a depressed bowl with an elevated rim. Subsequently, when sea level rose, new coral growth began in the optimal water depths above this jagged limestone rim. With every sea level fall and rise, erosion and new coral growth accentuated the relief between the central depression which became the lagoon and the rim which was now made up of towering stacked coral reefs. The modern looking atoll evolved in this manner.
A recent detailed study by Andre Droxler and Stephan Jorry compiling decades of research on the Pacific Ocean and Indian Ocean reefs demonstrates this elegantly.
The image above shows a portion of the Chagos atolls in the Indian Ocean. Notice how the rings of coral reefs have formed on a submerged flat 'mesa' or table land. These are the vestiges of the carbonate banks that formed during the Pliocene and later became the foundation for the modern atolls.
Great scientists get it wrong frequently too. Darwin erred in his too loyal adherence to the principle, 'Present Is The Key To The Past'.
He observed that some volcanic islands have coral colonies growing around them. He extrapolated this condition to the past and theorized that atolls began as reefs fringing volcanic islands. He couldn't imagine how otherwise corals could grow in the middle of the ocean without them having a shallow water foundation to colonize.
Uniformitarianism has been a very helpful paradigm in understanding many aspects of the past, but geologists have learned to apply it with caution. During the long 4.5 billion year history of our planet, critical combinations of atmosphere and ocean compositions and continental configurations coupled with the evolving biosphere have resulted in unique geologic products and processes which have no modern analogue.
Darwin's atolls offer us some lessons about our future too. Ten thousand years of relative climate stability has allowed civilization to flower, but has also lulled us into a dangerous complacency about nature and its permanence. The idyll of these atolls is ephemeral. Their very foundations are testimony to rapid environmental change and great dyings of marine ecosystems. Such changes await us in the future too, now hastened by our own agencies.
Several hundreds of years from now our descendants may well be looking on at a planet where many of Darwin's atolls have disappeared under the sea and our forests and croplands diminished by fire and dust. Will it be possible to modulate this coming change? For that, we must heed the signals from our geological past. Our present behavior will be the key to our future.
A more detailed write up about Andre Droxler and Stephan Jorry's work has been posted on the Rain to Rainforest Media website - If Darwin only knew: His brilliant theory of atoll formation had a fatal flaw.
No comments:
Post a Comment